Key points:

Court hearings being adjourned or held without defendant as video-links unavailable

Hundreds of union members expected to protest outside Parliament from midday

The strike means that all eight publicly run prisons across the state have been locked down today as guards walk off the job.

But the lockdown is also impacting on the state's justice system, preventing prisoners from being brought to court or into video-link suites inside prisons for their case hearings.

A Courts Administration Authority spokeswoman said matters would "still be listed as usual so there can be a judicial decision in each case about how to deal with the matter in the absence of the defendant".

Protests at Parliament House and in regions

Prison officers began their protest this morning at the remand centre in Currie Street before heading to Parliament House about 11:45am, chanting: "What do we want? Public prisons! When do we want them? Now!"

One prison officer, who refused to give his name, said the Government had been secretive about its privatisation plans.

"We're going to keep fighting this," he said.

"This is not the end of it, it's only the start of it."

Another said officers were worried about losing conditions.

"You only have to look at Mt Gambier [prison], which has been privatised how they're treated there," he said.

"It's a different story. That's what we don't want here."

Prison staff are also rallying in Port Augusta and Port Lincoln, but not in Mt Gambier.

Days of tense negotiation over protest

The protests come after days of tense negotiation between the Government and public service union, after Treasurer Rob Lucas sought court intervention to ensure public safety was maintained during the prison lockdown.

On Saturday, the South Australian Employment Tribunal ordered that minimum levels of staffing be maintained during the lockdown, which is expected to last up to 40 hours.

Mr Lucas said the Government respected the right for employees to take industrial action — but community safety was paramount.

"Ultimately, you don't want to have a situation where there might be in the worst possible set of circumstances rioting and escapes or anything like that, clearly prisons are dangerous environments," Mr Lucas said.

But Public Service Association spokesman Austin White said the Government went to the tribunal for the wrong reasons.

"We're unhappy that the Government appears to be using the tribunal to pursue its political agenda but we are content to proceed with our lockdown and our protest," Mr White said.

"The people of South Australia elected the Government to make decisions, not the union bosses … we've inherited a financial mess, we have to clean it up.

"We respect the right for employees to protest if they so wish, but it's not going to change the clear intention of the Government to get on with the job of … implementing the policies and programs that we outlined."